The present invention relates in general to tubular polymerization reactors and in particular to a new and useful control system for the polymerization of ethylene, which also includes features for optimizing the polymerization process. The control system of the invention is for the type of tubular reactor wherein heat removal is accomplished by externally cooling the reactor tube.
Low density polyethylene is an important basic polymer in a consumer oriented society. Over the years, its manufacturing processes have changed from the use of continuous stirred tank reactors to the use of tubular reactors for polymerization. This is in large part due to process economics.
The use of tubular reactors for ethylene polymerization instead of autoclave type reactors, is disclosed in Polyolefin Production Processes - Latest Developments, M. P. Sittig, Noyes Data Corp., Park Ridge, N.J., 1976.
Most low density polyethylene is made at high pressures ranging from 1,000 to 3,350 atm. pressure using a free radical initiator/catalyst such as peroxides, nitrogen compounds and metallic catalysts. The basic steps in the process are the initial compression of purified ethylene to the reactor pressure, introduction of free radical initiator/catalyst at some stage of compression, polymerization and removal of exothermic heat of reaction, reaction mixture let down, product separation and finishing. A tubular reactor for accomplishing at least some of these steps is disclosed in U.S. Pat. No. 4,008,049 to Clemmer et al.
Since ethylene polymerization is an exothermic process, exothermic heat must be removed. Such heat removal has been achieved by externally cooling the reactor tube, feeding a portion of cool ethylene feed as a starting material, to a high temperature reactor zone.
It is also important that ethylene feed to the reactor be 99.9% pure and acetylene free. Commonly acceptable limits for acetylene are less than 10 ppm. Ethylene gas feed should also contain less than 400 ppm oxygen since higher oxygen content results in lower reaction rate and thus lower polymer yield.
The iniator/catalyst, as noted above are free radicals and predominantly oxides and peroxides. From 0.01 to 10% by weight may be added, based upon desired polymer weight. Liquid catalyst is injected at multiple points in connecting lines between one or more compressers and the reactor, at a temperature higher than the polymer melting point.
Temperatures of 180.degree. to 200.degree. C. are commonly used in high pressure polymerization. Actual operating temperature depends on the temperature required for thermal decomposition of catalyst to provide free radicals for initiation of polymerization.
This reactor operating temperture is well above the critical temperature of ethylene. High operating pressures are therefore required to force the ethylenemolecules together and insure that free radicals will collide with an ethyl molecule during their short lifetime. The above mentioned operating pressure range is therefore used in the low density polyethylene process.
Major problems exist in known ethylene polymerization processes using tubular reactors.
One of these problems is that product quality is achieved by maintaining reactor temperature at a pre-specified level. That is, there is no direct product quality control. This approach results in significant fluctuations in product quality due to variations in reactor temperature caused by discharging reactor products through let down valves. A polymer-ethylene mixture is discharged from the reactor tube by opening and closing such a let down valve to lower reactor pressure intermittently. This causes pressure fluctuations of 200 to 1,000 Kg/Cm.sup.2.
Another problem is that the application of excessive cooling lowers reactor cooling zone temperature and results in polymer deposits on the tube wall. Pressure drops appear as a consequence and, in a worst case, the reactor tube may become clogged. This may result in damage to the equipment.
A further problem is that the amount of catalyst added is based on feed flow rate to the reactor. This is fixed for the operation since it is considered independent of feed flow rate as the catalyst control system does not account for variations in the feed flow rate.
A still further problem of the prior art is that significant amounts of operator attention is required during the process. This is because an operator must not only observe the process behaviour but must also adjust set points for temperature and catalyst, based on his knowledge of the process and a relationship of product quality vs. reactor temperature and catalyst flow rate vs. feed flow rate.